The invention relates to an arrangement for cooling the rotor of an electric machine, especially a turbine type generator, with a superconducting field winding which is surrounded by a co-rotating cold shield, and with a co-rotating mixing chamber which contains a phase mixture of a coolant fed in from the outside and from which a first coolant stream with a liquid coolant can be taken for cooling the field winding and a second coolant stream with an at least partially evaporated coolant for cooling the cold shield, and wherein the first coolant stream conducted through the field winding is returned to the mixing chamer and the second coolant stream conducted through the cold shield is returned from the machine.
A turbine type generator with a cooling arrangement of this kind is known from the report "EPRI TD-255, Project 672-1, Final Report", August, 1976, pages 45 to 48, with the title "Superconductors in Large Synchronous Machines". The coolant required for the cooling is conducted from a refrigeration machine centrally through the rotor shaft in liquid condition and at low pressure via a rotating coupling and is fed into a mixing chamber provided there near the axis. The corresponding feed point is designed as a Joule-Thomson device, so that a two phase mixture of liquid and gaseous coolant is formed in the mixing chamber. Due to the centrifugal forces acting on this two phase mixture during the rotation, the coolant vapor positions itself in regions near the axis and the coolant liquid in regions away from the axis of the mixing chamber. From the mixing chamber, a first coolant stream with liquid coolant is pumped via radial canals into a coolant bath in which the superconducting field winding is arranged. In the coolant bath, the heat dissipation of the winding is given off to the coolant, which is conducted back into the mixing chamber via further radial canals. The heat carried along in this process causes partial evaporation of the coolant. For cooling the cold shield, a second coolant stream with coolant vapor derived from the regions of the mixing chamber close to the axis is provided. The cold shield is in general a co-rotating damper shield between the field winding and a stationary stator winding of the machine surronding the former. After the second coolant stream has been conducted through cooling canals which are connected to the damper shield in a heat transfer manner, it is conducted out of the rotor via a further rotating helium coupling near the rotor axis and returned to the refrigeration machine. The necessary pumping action for developing the flow of the first and second coolant stream is brought about by the so-called self-pumping effect. There, the coolant is accelerated in radially outward-leading canals due to centrifugal forces and can thus be pumped into the field winding and the cold or damper shield, respectively. Since it is warmed up in these parts due to the heat dissipation occurring or by heat transfer from the outside, its specific gravity is decreased accordingly. Thus, a pressure gradient develops between the entrance and exit points of the coolant, which is sufficient for returning the coolant to regions near the axis.
However, in the known machine, the permissible damper losses are limited by the available quantity of vapor. In the event of disturbances, for instance, in case of sudden load changes, unbalanced load or short circuits, however, the damper loss can increase in step fashion. The amount of coolant of the second coolant stream available for removing this dissipation may then be too small, however, to prevent an excessive temperature rise of the cold or damper shield. At this temperature rise of the cold or damper shield also reacts back on the superconducting field winding, the danger exists that the field winding could heat up excessively.